Many of the wonderful properties of earth floors, including their strength and water resistance, derive from the oil used to harden and bind them – I find it useful to think of earth floors as linoleum made with earth, instead of cork and sawdust. Linoleum was originally hardened with boiled linseed oil, but linseed isn’t the only oil that works for earth floors. The main types of oil that have been tried for earth floors are boiled linseed, raw linseed, hemp, and tung (note that boiled linseed oil actually contains drying agents, and you should source oil that is free of heavy metal dryers). Sometimes resins are also included in oil blends used for earth floors. While each type of oil has its adherents for earth floors, it seems that there is little data that may be used to compare them – there isn’t even that much anecdotal evidence, other than that they can all work to some extent.
Because it haven’t seen it done elsewhere, I tested hardness of several oil and solvent combinations using an improvised Brinell hardness test. Brinell hardness is a simple test performed by pressing a 10mm ball bearing into a sample at a set pressure (usually 220 lbs) and time, and measuring the width of the resulting indentation. I used an improvised setup (inspired by this discussion), and the sample size is small – but despite a lack of scientific rigor some very striking trends can be seen. I’ll summarize the results first, but the actual measurements are presented in the table below.
- Tung oil has very high initial and later strength, but has terrible penetration, and must be cut with thinner 25-50% in order to be usable, or it can blended with other oils.
- Boiled linseed oil had relatively low 10-day strength, but gained a lot of strength with curing and was second only to tung oil in terms of hardness after 3 months. Mixing it with thinner reduced initial and probably late strength (the sample cracked and failed during the 3 month test). However it was clear that more oil-thinner mix could be applied to the sample, so the lesson is that when thinner is used, make sure to use extra coats of oil to ensure good saturation. Boiled linseed oil had moderate to poor penetration, and needs to be either heated (safely?!) or usually cut with thinner.
- Raw linseed oil had slightly better initial strength than boiled linseed, but gained virtually no strength over the next 90 days, making it one of the weakest samples after 90 days. Raw linseed oil has moderate penetration.
- Hemp oil has excellent penetration, and reasonable 10-day strength, but does not appear to gain strength very much over time, certainly not when compared to boiled linseed oil.
- Blending oils can capture the benefits of each. A blend of 1 hemp: 1 tung: 1 boiled linseed, diluted 10% with citrus solvent had very good strength (though not quite as good as tung oil and thinner alone), as well as intermediate penetration. The resulting hardness of 1.81 is better than that reported for Swedish pine flooring (1.6) but much lower than most hardwoods (e.g. black cherry is 3.6, whereas Swedish birch flooring is 2.6).
- Note that the boiled linseed oil I used was old, and may have suffered from some oxidation. Even so it appears to be one of the best oils for earth floors, though blending even a little tung oil into it will improve the resulting hardness. The raw linseed oil wasn’t fresh either.
| Oil | Comments | Penetration (1=best, 8=worst) | Hardness 10 days | Hardness 90 days |
| 100% Boiled Linseed | strong odour, intermediate penetration | 5 | 0.94 | 1.38 |
| 100% Hemp | mild odour, excellent penetration | 3 | 1.06 | Sample broke (weak) |
| 75% Tung / 25% Thinner | Poor penetration. Smell is nutty and smoky, reminiscent of chipotle | 8 | 1.71 | 2.05 |
| 100% Raw Linseed | intermediate odour, excellent penetration | 2 | 1.03 | 1.04 |
| 75% Boiled Linseed / 25% Thinner | best penetration | 1 | 0.85 | Sample broke (weak) |
| 75% Boiled Linseed / 25% Tung | Poor-Intermediate penetration | 7 | 1.01 | 1.61 |
| 75% Hemp / 25% Tung | intermediate penetration | 6 | 1.24 | 1.49 |
| 30% Boiled Linseed / 30% Tung/ 30% Hemp / 10% thinner | intermediate penetration | 4 | 1.37 | 1.81 |
| White pine | Trim grade | 0.97 | ||
| Pine Flooring | Value from internet | 1.6 | ||
| Spruce-Pine-Fir | Construction lumber | 1.24 | ||
| Cedar | Home depot | 0.77 | ||
| Black Cherry Flooring | Value from internet | 3.6 |
Caution
Drying oils are very reactive, and oily rags are prone to catching fire when left bunched up (sponges are even worse, and should be avoided). Immediately after oiling a floor all rags, rollers etc should be spread out outdoors – otherwise the photo you take of your freshly oiled floor might be the last photo of your house before it burned down.
A few tips
- Get as many coat on as you reasonably can in one day. 3-4 coats is is doable depending on how heavily you apply each one, the first coat can be poured on and moved around with a roller or a squeegee.
- Wait until one coat is completely penetrated before applying the next one (initially the coats can be applied almost immediately, later coats may take hours to finish penetrating).
- Wipe off any excess oil after a few hours, don’t leave it overnight or it will gel.
- Your freshly oiled floor is incredibly fragile – use bare feet or socks and roll up pant legs. Use soft rollers or brushes. Try to wait until oil has fully penetrated before crossing it for the next coat.
- Read Sukita’s book.
You did a wonderful job: thank you so much !
So, I want to share the following: some 37 years ago I learned from a then 75-years old builder how they made performant traditional earth floors. It’s knowledge from his father and his grandfather. They mixed boiled lindseedoil already into the earth mixture, as well as ashes from the local blacksmith’s fire. If not available, then ashes from the wood stove. (They contain potassium, which in my opinion cause at least a partial saponification of the linseedoil, so it binds better with the moistened earth). Also a part of hydrated lime (NOT hydraulic lime!) was added into this mixture, if available of course, as it was in those times. This gives really very strong floors, like those used as traditional treshing floors.
Wow, good information! Any tips on ratios? How much linseed, how much ash, relative to the earth mixture? From what I know of adding lime to clay, the amount of hydrated lime would be very important (and I do wonder if it’s necessary). I love this, thank you.
I haven’t liked the smell of the oils. The best result I’ve had is in making the final 1/4″ coat, mixing Vermont Natural’s PolyWhey water-based floor finish instead of water. Sets up really hard, light color, no smell.
Thank you for your suggestion. I have had people react strongly to the smells and will look for this product.
So Darrell, you’re saying you just mix the fine finish layer with that product instead of water and then don’t put a coat on the top?
G’day from Australia,
Thanks for sharing this valuable info Mike, you are a legend!
I’m doing a few earth floors in my place, I’ve started with the smallest – the study – about 3m x 3.5m. I’ve started sealing with raw linseed oil, did 5 flood coats 2 days ago, another yesterday, I heated the oil to about 100 degrees Celsius or 200 in your language, mainly to thin it.
The smell is very intense as the oil dries, is guess it oxidizing, is this giving odd nasty gasses we should be avoiding, it is making me a bit light headed even with good ventilation. and my sample took months for the smell to go!
You hear everyone in green circles say to stay away from the boiled because of the chemical drying agents, are they as really as bad as they say, to my mind a shorter time drying even with the chemical might save being exposed to the natural gasses of the raw for months on end??
I am considering boiling my raw oil for the final couple of coats to help it harden and hopefully make the smell not as nasty – we plan to move in in 2 or 3 months time, any thoughts?
Thanks for time
Andrew
Hi Andrew, sorry this is late in coming. my understanding is that the while the odour is strong it is relatively non-toxic (some people have sensitivities). I also find the odour strong, and plan oiling in a season when good ventilation is possible for a month or more! You can buy more natural boiled linseed oils that don’t use heavy metal dryers, I’ve never tried boiling it myself.
You’re probably already moved in at this point, so advice may be irrelevant, but I’ve started sealing earth floors with Le Tonkinois tung-oil based varnish. I think this might help contain the oil smell, but also smells for a few weeks! Also it adds some strength and improves cleanability of the floor. Because it penetrates it basically becomes part of the floor, so peeling is not a concern. More traditionally an earth floor would be finished with an oil-wax, which also helps seal the surface.
Hi Mike,
Really appreciate the information on the different oils.
I’m also about to lay an earthern floor and I have been researching, as best I can, different oil mixes. I have quite a large surface to work on as it is a 4 bedroom cob house, so the oil is quite an expense. I’m intrigued by the Le Tonkinois product, although it is significantly more expensive than using raw tung oil and linseed oil. Would you say the extra cost is worth it? Have you done any testing? Many thanks.
I use the Tonkinois varnish as a final finish after the oil has cured, in place of wax. Is a great finish, depending on what you’re looking for. The oiling is the same though.
Hi Mike,
Thank you for doing a great experiment & write up.
I have no experience with sealing clay and wondered if I could combine a few of your methods used above.
I have cast a large earthen fire hearth with my left over clay from plastering my walls. Approx 25mm thick. It has fully dried and successfully no cracks.
I would be extremely grateful for your / anyones comments on my thoughts on sealing.
I thought of using the coats you mentioned above in a sequence decreasing absorption but increasing hardness.
Using:
Boiled linseed with 25% citrus thinner for absorption on initial 1 or 2 coats. (good absorption / poor hardness)
Followed by 100 % boiled linseed 1 or 2 coats. (medium absorption / medium hardness)
Followed by tung oil with 25% thinner. (poor absorption / good hardness.)
Can you see any problems using this method?
Is there a wax anyone would recommend? And would you add it to the tung oil or apply separately?
Thank you for your time.
Joseph in Ireland
That sounds reasonable. You’d add the wax after the oil cures.
Hi mike great article and extremely useful information. Will raw Flaxseed oil do in place of linseed oil, as in will it harden and protect the finish ? It is on an earth wall rather than a floor. Many thanks Tony
On a wall that might be ok, it lacks the strength you need for a floor. It should provide some water resistance etc. Do tests!
Thanks Mike, I used the flaxseed oil and it worked fine, strength is sufficient and waterproof too